Switching power supply units, also known as DC-DC converters, are widely used in various electric and electronic apparatuses. Stability in output voltage against a wide range of input voltage and load fluctuations is often demanded from such switching power supply units. Power supply units that select either a pulse width modulated signal or a pulse frequency modulated signal during switching operation, and perform switching using the selected signal in order to stably control output voltage are known.
For instance, a DC-DC converter that selects a pulse frequency modulated signal when a load current flowing through the load is below a predetermined value and selects a pulse width modulated signal, irrespective of a value of the load current, when a level of the DC voltage supplied to the load changes is disclosed in Patent Document 1. In the case where the load becomes increasingly heavy in such a DC-DC converter, the output voltage to the load decreases and it is controlled so that a pulse width modulated signal is selected as a switching signal when the output voltage is below a predetermined voltage value. In other words, the decrease in the output voltage triggers the selection of the pulse width modulated signal in order to supply more power to the load.
[Patent Document 1]
Japanese Patent Kokai Publication No. JP-P2004-96982A